The present invention relates to an internal combustion engine. In general, in four-cycle gasoline engines, an air/fuel mixture having a predetermined constant air/fuel ratio is introduced into the cylinders thereof following which compression, combustion, expansion and exhaust cycles are performed. In case the engine is loaded, the amount of suction on the incoming mixture is limited and controlled by a throttle valve. The amount of suction is reduced in proportion to the negative intake pressure generated at such times to thereby adjust or control the engine output power.
A pressure-volume graph (P-V graph) for such a known four-cycle gasoline internal combustion engine operated as noted above is shown in FIG. 1. The negative work portion of the complete cycle increases in proportion to the negative suction pressure as indicated by the hatched region in FIG. 1. This negative work causes the fuel consumption of the engine to increase as the engine is more heavily loaded.
The above-noted negative work is inherent to prior art gasoline engines which utilize a fixed air/fuel ratio. In particular, since partial loads on the engine are quite frequent, the increase in the fuel consumption for partial loads leads to an increase in the overall fuel consumption of the vehicle. Improvements in fuel consumption have been of utmost importance in recent years.
On the other hand, in a turbocharged engine, the higher the engine rotational speed, the greater is the engine output power which can be obtained. However, a turbocharged engine has an inherent defect in that a sufficiently high output power often cannot be obtained at low rotational speeds. It is desirable that maximum torque be produced at intermediate rotational speeds of an automotive engine, that is, at approximately 50% of the maximum rotational speed of the engine. Accordingly, to obtain such torque characteristics, it is required that the fuel supply be restricted when the intermediate engine rotational speed is reached to thereby reduce the torque of the engine. For this reason, the full capabilities of the turbocharged engine have not yet been fully utilized.
A turbocharger for an automotive engine usually employs a velocity-type compressor in which the outlet pressure thereof is in proportion to the second power of the emission flow used to drive the turbocharger's turbine. If the turbocharger is used in an automotive engine, the outlet pressure thereof will be in proportion to the second power of the engine rotational speed.
In FIG. 8, a curve (1) to (2) is a typical operational curve of a turbocharged engine. If a higher pressure ratio were attempted, the engine would operate in a surging region designated by the hatched area in FIG. 8 and, accordingly, it would be impossible to output compressed air. A curve (2) to (3) shows the amount of air produced and the pressure thereof. Attempts at increasing the pressure of the output air beyond the curve (2) to (3) result in damage to the turbocharger by an excessively high rate of rotation.
At the idle speed of the engine, for example, at a speed of about 500 rpm, little turbocharger pressure is, as shown at point (4), generated. For middle speeds or, correspondingly, the middle range of gas flow, thereof, a significant amount of turbocharged pressure is generated. For middle engine speed or gas flow values, generation of turbocharged pressure at about a quarter of the maximum output is possible. For example, at 1000 rpm a pressure ratio corresponding to the point (11) in FIG. 8 is attained. At the maximum gas flow or engine speed values, for example, 2000 rpm, an increase by a factor of four times relative to that at 1000 rpm is possible so that the turbocharged pressure increases up to the point (5).
On the other hand, the torque of the turbocharged engine increases to the point (6) within which the amount of fuel mixture supplied increases in proportion to the turbocharged pressure as indicated by the dotted curve in FIG. 8 whereby a torque property as shown by curve (7) to (6) is obtained. A high torque property as shown by curve (7) to (6) may be usefully employed in ship engines in which a propeller is driven. However, an automotive engine requires torque characteristics as shown by curve (7), (8) to (9) where high torques are provided at the middle engine speeds. Accordingly, as the engine rotational speed increases, the amount of fuel to be fed should be decreased to thereby realize the torque characteristics as indicated by the curve (8) to (9). In this case, since the amount of gas emitted increases in proportion to the engine speed, even if the amount of fuel to be supplied is restricted, the curve will be raised up to point (10) resulting in an unduly high increase in the engine compression pressure.
As illustrated above, in a modern turbocharged engine designed for automotive use, even though the turbocharger is able to attain the supercharged pressure shown as at point (5) in FIG. 8, to obtain the actual desired torque characteristics (7)-(8)-(9), only a supercharged pressure up to point (11) is in fact used resulting in the usage of only a quarter of the maximum capacity of the turbocharger.
Accordingly, an object of the present invention is to provide an internal combustion engine solving the above noted problems inherent in various conventional engines.
Another object of the invention is to improve the fuel consumption at partial loads of, for example, gasoline engines by decreasing the negative work at partial loads to as small a value as possible.
Still another object of the invention is to provide a turbocharged engine for automotive use having a high output power and a low fuel consumption, in which the maximum possible output of the supercharger is employed at the middle speeds of the engine, the amount of gas flow and thermal energy supplied to the exhaust gas turbine of the turbocharger are maintained constant and the amount of air and pressure outputted from a compressor of the turbocharger is also maintained constant whereby the maximum ability or performance of the turbocharger is utilized.